Optical applications of thin oxide films start from single layer type heat reflecting glasses and antireflection films and extend to various fields including, for example, multi-layer type antireflection coatings, reflection enhancing coatings, interference filters and polarizing films, which are designed to permit lights having certain specific wavelengths to reflect or pass selectively therethrough. Further, a study has been made to insert a transparent electroconductive film or a film of e.g. metal or electroconductive ceramics having various functions such as electroconductivity and heat reflection properties as a part of a multi-layer film to obtain a multi-layer film having a function such as an antistatic, heat reflecting or electromagnetic wave shielding function provided.
The spectral characteristics of a multi-layer film are optically designed by using refractive indexes n and thicknesses of the respective layers, as parameters, and it is common to employ a combination of a high refractive index film and a low refractive index film. To realize excellent optical properties, the larger the difference in the refractive index between the high refractive index film and the low refractive index film, the better. As such a high refractive index film, titanium dioxide (n=2.4) cerium dioxide (n=2.3), zirconium dioxide (n=2.2), niobium pentoxide (n=2.1), tantalum pentoxide (n=2.1) or tungsten trioxide (n=2.0) is, for example, known. Further, as a low refractive index film, silicon dioxide (n=1.46) or magnesium fluoride (n=1.38) is, for example, known.
Such films can be formed, for example, by a vacuum vapor deposition method or a coating method. However, by such a film-forming method, it is difficult to form a uniform film over a substrate having a large area, and when a substrate having a large area, such as a glass for buildings or automobiles, CRT, or a flat display, is required, sputtering is used in many cases. Among various sputtering methods, DC sputtering utilizing direct current discharge is most suitable for forming a film over a large area.
When a high refractive index film is to be formed by DC sputtering, it is common at present to employ so-called reactive sputtering wherein a metallic target having electroconductivity is subjected to sputtering in an atmosphere containing oxygen. However, there has been a problem that the film-forming speed of a thin film obtainable by this method is very slow, whereby the productivity is poor, and the cost tends to be high.
To solve such a problem, it has been proposed to use an oxide ceramic (sintered body) as a target. However, oxide ceramic usually has no electroconductivity, whereby DC sputtering has been difficult.
Further, recently, a sputtering target is required to have a complex shape, and a highly efficient planar target having the target thickness partially changed, is required. By a method for obtaining a sintered body by a common sintering method, it is difficult to produce a target having a complex structure or various shapes, and such a target is prepared by a long process including steps of mixing starting materials, sintering, processing and bonding, whereby substantial jigs are required for its production.
In sputtering over a glass sheet with a large area for buildings, the film-forming speed is increased by applying a high power for sputtering to increase the productivity, whereby cooling of the target tends to limit the film-forming speed, and further troubles such as cracking of the target, peeling, etc., are likely to occur.
A new magnetron type rotary cathode is known wherein such drawbacks have been overcome (JP-A-58-500174). This is of a type wherein a magnetic field generating means is provided inside of a cylindrical target, and sputtering is carried out while rotating the target and cooling the target from inside. By the use of such a cylindrical target, a large power per unit area can be applied as compared with a planar type target, whereby film formation at a high speed is said to be possible.
Preparation of a target material on a cylindrical target holder has heretofore been commonly carried out when the target material is a metal or alloy. In the case of a metal target, multi-layer film coatings of e.g. its oxide, nitride, carbide, etc. are formed in various sputtering atmospheres. However, it has had drawbacks that the coating films are likely to be damaged by different types of atmospheres, whereby films having desired compositions can hardly be obtainable, and, in a case of a low melting metal target, the target is likely to undergo melting when the power applied is excessive. Under these circumstances, a ceramic target material has been desired. A method has been proposed in which a ceramic sintered body is formed into a cylindrical shape and bonded to a substrate by means of indium metal. However, the method is difficult and costly.
JP-A-60-181270 proposes a process for producing a ceramic sputtering target by spraying. However, the process has had problems that the sprayed coating can not be made sufficiently thick, as the difference in thermal expansion between the ceramics and the substrate metal is large, and the adhesion tends to deteriorate by thermal shock during its user thus leading to peeling.
JP-A-62-161945 proposes a process for producing a non-electroconductive ceramic sputtering target made of various oxides by water plasma spraying. This target is a target for radio frequency (RF) sputtering, and the target itself is an insulating material. Further, this target has had drawbacks that, unless some measures such an undercoating is taken, it is likely to undergo cracking or peeling as the temperature rises during sputtering, whereby film formation under a stabilized condition tends to be difficult. Further, there has been a drawback such that the film forming speed is very slow.
It is an object of the present invention to provide an electroconductive sputtering target which can be formed into any desired shape and which is capable of forming a high refractive index film at a high speed by DC sputtering, a process for its production, and a method for forming a high refractive index film using such a target.